1. Field of the Invention
The present invention relates to an image heating apparatus adapted for use as a heat fixing apparatus in a copying machine or a printer, and a heater adapted for use in such image heating apparatus.
2. Description of the Related Art
In a heat fixing apparatus for a copying machine or a printer, there is commercialized an apparatus of a configuration having, as disclosed in Japanese Patent Application Laid-open No. S63-313182, a flexible sleeve, a ceramic heater in contact with an internal surface of the flexible sleeve, and a pressure roller constituting a nip portion with the ceramic heater through the flexible sleeve, in which a recording material bearing a toner image is conveyed by the nip portion to heat fixing the toner image onto the recording material. Such heat fixing apparatus (called film heating type), having a very low heat capacity, has advantages of a quick warning up to a fixable temperature thereby providing a short print waiting time, and a low electric power consumption in a stand-by state waiting for a print command.
The flexible sleeve is made of polyimide or stainless steel. Also the ceramic heater is formed by printing a heat-generating resistor principally constituted of silver or palladium on a plate-shaped ceramic substrate excellent in heat resistance, thermal conductivity and electrical insulation such as of alumina or aluminum nitride. A temperature of the heater is controlled by controlling a current supply to the heat-generating resistor, based on a temperature detected by a thermistor maintained in contact with the ceramic heater.
Such fixing apparatus, though being excellent in the quick-starting property because of its low heat capacity, is associated with drawbacks because of such low heat capacity. In case the longitudinal length of the recording material is relatively short in comparison with the longitudinal length of the heater, an amount of heat taken away from the heater is different significantly, in the nip portion, between a sheet passing area passed by the recording material and a sheet non-passing area not passed by the recording material, so that the temperature of the sheet non-passing area, where the heat is not taken away by the recording material, is gradually elevated as the sheets are passed one by one. Thus there tends to result a temperature elevation phenomenon in the sheet non-passing area, which becomes more marked in the film heating system of low heat capacity. Since an excessive temperature elevation phenomenon in the sheet non-passing area causes a thermal deterioration of the components of the fixing apparatus thereby leading to a reduction in the service life of the apparatus, there have been proposed a heater configuration and a control method for the fixing apparatus for solving such drawbacks.
Japanese Patent Application Laid-open No. 2000-162909 proposes a method of reducing the aforementioned temperature elevation in the sheet non-passing area, utilizing a heater 700 of a structure as shown in FIG. 12A. Also FIG. 13A shows a heater driving circuit 70.
A heater 700 shown in FIG. 12A is provided with plural heat generating patterns 701a, 701b having different heat generating areas in the longitudinal direction of a ceramic substrate 704, and also with current-supplying electrodes 702a, 702b and a common electrode 703 for independent current supplies to the heat-generating patterns.
A heater driving circuit 70 shown in FIG. 13A is an example of a driving circuit for controlling the current supply to the heater 700. A thermistor 50 is contacted with the heater 700 or provided in the vicinity thereof, and supplies a CPU 71 with a detection result of the temperature of the heater 700. The CPU 71 controls turn-on timings of triacs 72a, 72b so as to execute a desired temperature control, based on the temperature detection result by the thermistor 50. The CPU 71 is capable of determine a turn-on ratio of the triacs 72a, 72b and can execute the temperature control with a desired heat generation ratio. Also a safety element 60 (temperature fuse or thermo switch) for preventing an excessive temperature elevation of the heater 700 is provided serially in the current supply line and is contacted with the heater 700 or provided in the vicinity thereof, and such safety element 60 is activated in a thermal uncontrollable state of the heater 700 to cut off the power supply to the heater 700.
In the fixing apparatus equipped with the heater 700 of FIG. 12A and having a reference position of sheet passing at the center of the longitudinal direction, in case of fixing a recording material of a relatively large longitudinal length (hereinafter called large-sized sheet), a current is given between the electrodes 702b and 703 to heat the heat generating pattern 701b, and in case of fixing a recording material of a relatively small longitudinal length (hereinafter called small-sized sheet), a current is given between the electrodes 702a and 703 to heat the heat generating pattern 701a, thereby reducing the temperature evaluation in the sheet non-passing area.
Also Japanese Patent Application Laid-open No. 2000-250337 proposes a similar heater configuration, in which three heat-generating patterns are independently activated as shown in FIG. 12B. In this case, a heater 800 is provided on a ceramic substrate 804, heat-generating patterns 801a, 801b, 801c, current-supplying electrodes 802a, 802b, 802c and a common electrode 803 and is driven by a heater driving circuit 75 shown in FIG. 13B, whereby each heat-generating pattern can be independently activated.
Also Japanese Patent Application Laid-open No. H10-177319 proposes a fixing apparatus employing a heater capable of forming an arc-shaped heat generation distribution by a multi-step heat generation control according to various sheet sizes, thereby suppressing the temperature elevation in the sheet non-passing area within a certain range while securing the fixing property.
A heater 900 shown in FIG. 12C is provided with plural heat generating patterns 901a, 901b having different heat generating distributions in the longitudinal direction of a ceramic substrate 904, and also with current-supplying electrodes 902a, 902b and a common electrode 903 for independent current supplies to the heat-generating patterns. The heat generating pattern 901a has a width which is widened in plural steps from an approximate center in the longitudinal direction toward end portions to reduce the resistance per unit length, thereby providing a convex heat generation distribution with a peak heat generation at the center of the longitudinal direction under a current supply, while the heat generating pattern 901b has a width which is made narrower from the approximate center in the longitudinal direction toward end portions to increase the resistance per unit length, thereby providing a concave heat generation distribution with a bottom heat generation at the center of the longitudinal direction under a current supply.
With the heater 900, a smooth slope can be obtained in the heat generation distribution in the longitudinal direction, by incorporating the heater 900 in a heater driving circuit 70 shown in FIG. 13A and executing a control with a turn-on ratio of the triacs 72a, 72b determined by a CPU 71. In the fixing apparatus equipped with such heater 900 and having a reference position of sheet passing at the center of the longitudinal direction, it is possible to control the temperature elevation in the sheet non-passing area and the fixing property at the same time in more strict manner, by selecting the turn-on ratio of the triacs 72a, 72b within a range from 10:10 to 10:0 according the longitudinal length of the recording material.
However, in such fixing apparatus of film heating type utilizing such ceramic heater, in so-called uncontrollable situation of the fixing apparatus caused for example by a failure of the triac therein, the heater may show an excessive temperature increase and the ceramic substrate may be cracked by a thermal stress applied to the heater before the safety element (temperature fuse or thermo switch) can function. Also depending on the manner of cracking of the ceramic substrate, a dielectric strength cannot be satisfied between a resistance circuit (AC) side (primary side) including the heat generating pattern and a temperature sensor circuit (DC) side (secondary side) for heater temperature detection and the secondary circuit may be destructed by a current leaking to the main body of the image forming apparatus equipped with the fixing apparatus.
A thermal stress σ applied to a cross section of the substrate is represented, in case the temperature distribution is symmetrical within the cross section of the substrate, by a linear thermal expansion coefficient ε and a Young's modulus E of the substrate and a temperature difference ΔT within the substrate, which is dependent on the thermal conductivity thereof, by a following equation:σ=ε·E·ΔT
However, in case the temperature distribution is asymmetrical, it no longer is simply proportional to the temperature difference ΔT because a bending moment is applied to the substrate, and the tensile stress generally becomes larger at the bending side of the substrate. A breakage occurs when such tensile stress exceeds the bending strength (breaking strength) of the substrate.
For example, in case of a heater bearing a heat-generating pattern along the longitudinal direction on a surface of an alumina substrate having a length of 370 mm, a width of 10 mm and a thickness of 1 mm, a largest thermal stress is known to occur in a cross section in the direction of width (shorter side) of the substrate. Therefore, the breakage of the heater by the thermal stress can be considered to depend largely on the temperature distribution in the direction of width (shorter side) of the substrate.
In a heater with prior plural drives, namely in a heater in which plural heat generating patterns are independently driven by plural triacs, in case of a thermal uncontrollable of the heater by a failure in a triac, the temperature distribution increases asymmetry in the cross section in the direction of width of the substrate, and a margin to the heater breakage is limited because of a strong tensile stress functioning at the same time.
For example, in the heater 700 shown in FIG. 12A, since the heat generating pattern 701a is formed in an asymmetric area with respect to an approximate center CL in the direction of width (shorter direction) of the substrate (hereinafter represented as approximate shorter side center of the substrate), a failure in the triac 72a shown in FIG. 13A induces a large asymmetry in the temperature distribution in the cross section in the direction of width of the substrate, thereby showing a limited margin for the breakage.
In the heater 800 shown in FIG. 12B, though the entire heat generating patterns are formed symmetrically with respect to the approximate shorter side center CL of the substrate, since each heat generating pattern can be driven independently, a failure in the triac 77a or 77b shown in FIG. 13B induces a large asymmetry in the temperature distribution, thereby showing a limited margin to the breakage.
Also in the heater 900 shown in FIG. 12C, though the entire heat generating patterns are formed symmetrically with respect to the approximate shorter side center CL of the substrate, a thermal uncontrollable in one of the heat generating patterns 901a, 901b induces a large asymmetry, thereby showing a limited margin to the breakage.